1. Field of the Invention
This invention relates generally to a fuel cell and, more particularly, to a fuel cell that includes specially positioned openings for allowing a controlled amount of an anode exhaust gas flow to be mixed with a cathode input gas flow to provide combustion in the cathode flow channels for heating the fuel cell during start-up.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. A PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for an automobile may have two hundred stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack. For the automotive fuel cell stack mentioned above, the stack would include about four hundred bipolar plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It is desirable during certain fuel cell operating conditions, such as fuel cell start-up, low power operation, low ambient temperature operation, etc., to provide supplemental heat to the fuel cells to maintain the desired temperature within the fuel cell stack for proper water management and reaction kinetics purposes. Particularly, the MEAs must have a proper humidification and the cells must have a minimum temperature to operate efficiently.
The anode stoichiometry is typically greater than one, for example 1.02, during fuel cell operation so that hydrogen is properly distributed to the MEAs. Therefore, excess or unused hydrogen is generally available at an anode exhaust that must be properly dispersed or contained because it is combustible. It has been proposed in the art to combine this anode exhaust gas with the cathode input air to provide combustion either in the anode channels or the cathode channels to provide the desired supplemental heat for cold starts, low temperature or low power operating conditions, cabin heating, partially humidifying the cathode input gas, etc.